1. Field of the Invention
The present invention relates to a vibration isolating device and, more particularly, to a vibration isolating device which not only protects building structures from sharp earthquake shocks but also effectively absorbs moderate and minor earthquake shocks and weak vibrations produced by external forces at ordinary times.
2. Description of the Prior Art
In recent years a variety of vibration isolating devices have been developed from the viewpoints of protecting personnel and objects accommodated in building structures from damage by earthquakes and saving construction materials for absorbing vibrations acting on the building structures themselves. FIG. 1 shows a typical prior art example, in which an isolator 3 formed by laminated rubber plates is disposed between the base 1 and the foundation 2 of a building and a plurality of main dampers 4 formed by steel rods are planted around the isolator 3 (although only one main damper is shown for the sake of brevity). This vibration isolating device is arranged so that when the base 1 and the foundation 2 of the building are displaced horizontally, by vibration, in excess of a predetermined value, the vibrational energy will be absorbed by elastic and plastic deformations of the main dampers 4, thereby damping vibrations which are transmitted to the building itself.
The relationship between shearing force Q acting on the main damper 4 and its horizontal displacement .delta. is along with hysteresis curves of its elastic and plastic deformations in FIG. 2. The segment OA indicates no-load displacement of the damper 4 until its top end portion strikes against one inner surface 5a of an engaging hole 5 in the base 1 of the building. The segment AB indicates an elastic deformation of the damper 4 and the segment BC its plastic deformation. The vibrational energy is mostly consumed by the plastic deformation of the damper 4 indicated by the segment BC. The segment CD indicates an elastic deformation of the damper 4 in a direction in which it is restored upon removal of the shearing force Q. The segment DE indicates no-load displacement of the damper 4 until its top end portion strikes against the other inner surface 5b of the engaging hole 5. The segment EF indicates an elastic deformation of the damper 4 and the segment FG its second plastic deformation. This plastic deformation also consumes vibrational energy and damps the vibration. The segment GH indicates an elastic deformation of the damper 4 in the direction of its restoration, the segment HI no-load deformation of the damper 4 until its top end portion hits again against the inner surface 5a of the engaging hole 5, and the segment IJ an elastic deformation similar to that indicated by the segment AB. The main damper 4 is disposed with its top end portion spaced apart from the engaging hole 5 as indicated by a, and hence will not engage the hole 5 when its displacement is small.
Accordingly, the above-described vibration isolating device is intended primarily to cope with relative strong shocks which are produced by great earthquakes, as shown in FIG. 3, and no particular consideration is paid to the vibration isolating or preventing action (hereinafter referred to as a vibration damping action) against vibrations by moderate and minor earthquakes and strong winds and vibration by traffic and similar slight vibrations. That is to say, in the case of a big earthquake which will cause the displacement of the main damper 4 to exceed .delta..sub.1 in FIG. 2, the damping action will be performed by the plastic deformation of the damper 4, but when the displacement is below .delta..sub.1, the damping action will not be effectively achieved.
At present so-called intelligent buildings are becoming increasingly popular, and many precision apparatus and equipment such as electronic computers are installed in such an intelligent building, and there is a strong demand for a vibration isolating device which is capable of effectively damping moderate and minor vibrations or shocks as well. Table 1 shows uses of quake-free buildings and damping capabilities required therefor.
TABLE 1 ______________________________________ Buildings in Buildings in which which preci- Ordinary electronic sion equip- quake-free computers are ment are Use buildings installed installed ______________________________________ Great Shock waves Earthquake should be suppressed. Moderate Appropriate attenuation should be earthquake maintained. Strong wind Vibration Shock waves by traffic should be Slight suppressed. vibration at ordinary times ______________________________________
It has also been suggested to dispose a buffer as of rubber in the engaging hole 5 of the base 1 so that medium and small vibrations in the horizontal direction by moderate and minor earthquakes and slight vibrations in the horizontal direction at ordinary times are absorbed and damped by the isolator and the buffer. However, our experiments have revealed that this method is still defective in that since the buffer is packed in a circular form in the limited gap between the steel rod of the main damper 4 and the building structure, there is severe limitations on the amount of buffer packed and the area contributing to the buffer function; therefore, no sufficient energy absorbing capability is provided.